1. Field of the Invention
The present invention relates to a capacitive load driving circuit and method, a liquid droplet ejection device, and a piezoelectric speaker driving device, and in particular, to a capacitive load driving circuit and method, a liquid droplet ejection device, and a piezoelectric speaker driving device which stably operate a capacitive load.
2. Description of the Related Art
A driving circuit of an inkjet head outputs an analog driving signal to a piezoelectric element for ejecting an ink droplet from a nozzle of the inkjet head, and thereby causes an ink droplet to be ejected from the nozzle which is provided so as to correspond to that piezoelectric element. A piezoelectric actuator is a capacitive load, and there are the following three problems in driving it.
First, the driving circuit is an analog amplifying circuit (a Class B amplifying circuit). Therefore, there is the problem that, if a large number of nozzles are driven simultaneously, a large amount of heat is generated and the energy efficiency is poor (30 to 40%). Second, if there are many nozzles which are driven simultaneously, the impedance of the load decreases and the waveform becomes weak (dull). As a result, the jetting characteristics of the head are affected. Third, in terms of packaging, a large heat sink is needed in order to dissipate heat. Therefore, at the time of realizing high-speed printing by using a large number of nozzles, there are the problems that the packaging surface area increases and costs increase.
Thus, there has been proposed a driving waveform generating device for an inkjet print head which stores, in advance, a waveform data group for generating driving waveforms, selectively reads-out at least one waveform data to be used from among this waveform data group, carries out a predetermined arithmetic processing on the waveform data which is read-out, prepares a driving waveform, subjects the signal of this driving waveform to D/A conversion, and then amplifies and outputs it (see, for example, Japanese Patent No. 2940542).
There has also been proposed a driving circuit of an inkjet recording head, which the driving circuit is equipped with: storage means for storing driving waveform information for generating a driving waveform signal for each ink droplet diameter; plural waveform controlling means provided for the respective ink droplet diameters, and reading-out, from the storage means, driving waveform information corresponding to configurations of driving waveform signals which are to be generated, and successively outputting the information; plural waveform generating means which are provided for the respective ink droplet diameters, and which, after subjecting the driving waveform information, which is successively outputted from the waveform controlling means, to analog conversion, carry out integration processing so as to generate corresponding driving waveform signals; and driving means selecting, in accordance with the value of printing data, one driving waveform signal from among the plural driving waveform signals outputted from the plural waveform generating means, and applying it to a piezoelectric element (refer to Japanese Patent No. 3223891 for example).
Further, there have also been proposed head driving devices for recording devices in which a switching circuit for selecting a head driving channel, and an electric power amplifier, which supplies to this switching circuit electric power which drives the head, are connected by a wiring material (see, for example, Japanese Patent Applications Laid-Open Nos. 11-020151 and 11-020155).
In the head driving device for a recording device of JP-A No. 11-020151, the electric power amplifier has a negative feedback circuit. This negative feedback circuit leads a signal wire for negative feedback out from an input terminal of the switching circuit to the electric power amplifier, and the transmission system of the wiring material is inserted in the negative feedback loop. Further, in the head driving device for a recording device of JP-A No. 11-020155, the electric power amplifier has a feedback circuit. This feedback circuit leads signal wires for feedback out to the electric power amplifier from the ground point and from the point where one switch of the switching circuit and one channel of the head are connected. This switch is inserted in the feedback loop.
There is also proposed an inkjet head driving circuit (refer to Japanese Patent No. 3601450 for example) having a waveform generating circuit which generates a driving waveform signal, and an electric power amplifying circuit which, with the driving waveform signal being one input thereof, amplifies the driving waveform signal and outputs it to a piezoelectric element. In this circuit a feedback signal, which feeds-back the terminal voltage of the piezoelectric element, and an output signal from the electric power amplifying circuit are collectively the other input of the electric power amplifying circuit.
All of the above-described techniques can for the most part overcome the above-described first and second problems. However, they cannot overcome the third problem of the packaging surface area increasing due to the generation of heat.